Collision Theory & Maxwell–Boltzmann Distributions (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about collision theory and Maxwell–Boltzmann distributions.

The two are covered together because collision theory says particles need enough energy to react, and the Maxwell–Boltzmann distribution shows how that energy is spread across the particles in a sample.

Collision theory says a reaction only happens when particles collide with the correct orientation and with enough energy — the activation energy. The Maxwell–Boltzmann distribution shows how particle energies are spread across a sample at a given temperature, and how that spread changes with temperature, which is why it's the graph behind collision theory's energy requirement.

This video starts with collision theory and what makes a collision effective, then moves to the Maxwell–Boltzmann distribution and how the energy spread it shows explains why raising temperature increases the rate of a reaction.

For a reaction to happen, particles need to collide with the correct orientation and with energy equal to or greater than the activation energy. An ineffective collision is one where the particles are in the wrong orientation, or don't have enough energy, and they simply bounce off each other. The activation energy is the minimum energy particles need to react. Increasing the frequency of collisions increases the number of collisions with energy above the activation energy, which increases the rate of reaction.

A Maxwell–Boltzmann distribution curve shows how particle energies are spread across a sample at a given temperature — only a small proportion of particles have very low or very high energy, and most sit around the middle. Only particles with energy equal to or greater than the activation energy can react. When temperature increases, the curve flattens and its peak shifts to the right, meaning a greater proportion of particles now have energy equal to or greater than the activation energy, alongside particles colliding more frequently, further increasing the rate of reaction.

The increase in the proportion of molecules with energy greater than the activation energy has a bigger effect on the rate of reaction than the increase in collision frequency. And remember, activation energy is the energy needed to 'activate' the reactant particles so that a collision between them is effective.

A successful collision needs both the correct orientation and enough energy — energy equal to or greater than the activation energy. The Maxwell–Boltzmann distribution shows how that energy is spread across the particles in a sample. Raising the temperature flattens the curve and shifts more particles past the activation energy, which is why increasing temperature increases the rate of reaction.